Uranium and radon are naturally occurring radioactive elements. Radon is a heavy gas that is an intermediate product of uranium decay. Uranium present in rocks undergoes a sequence of alpha and beta decay through a number of unstable isotopes of various elements, ending, eventually, with a stable isotope of lead. Rock contains small, but variable, amounts of uranium, granite having relatively high levels. For this reason, uranium and radon levels tend to be high in areas with granite bedrock; however, other factors influence the actual levels of radon gas reaching the surface.
Production of radon results from the decay of radium-226, a heavier isotope in the sequence that begins with uranium-238, by the emission of an alpha particle. The isotope created by this process is radon-222, which is itself unstable and emits an alpha particle, forming polonium-218, the next isotope in the sequence. Radon-222 has a half-life of 3.8 days, which means that after this period, half of a sample of radon will have decayed.
Exposure to uranium and radon can have consequences for health. In contrast to the “enriched” uranium used in nuclear reactors, naturally occurring uranium-238 is only mildly radioactive. Under normal circumstances it does not present a threat, although it is chemically toxic if ingested. Radon is a more serious hazard as it is highly radioactive and, as a gas, can easily escape from bedrock and accumulate in homes, where it can be inhaled.
Both uranium and radon produce alpha particles. Although these particles cannot penetrate skin, if they come into contact with living tissue they can cause damage. When DNA is damaged it can lead to cancer. If radon is inhaled, it is likely to come into contact with lung tissue and long-term inhalation of radon in homes can result in chronic lung disease and lung cancer. Most lung cancer is caused by smoking, but radon is the second biggest cause.
Levels of radon in homes depend upon a number of factors in addition to the uranium content of the bedrock. While the gas will tend to rise toward the surface due to pressure differences, it can do so more quickly if there are fractures present in its bedrock source, and if the soil and subsoil above are very permeable. Moisture, however, slows its progress as it can dissolve in water. On reaching the surface, it can be drawn into homes, especially if the foundations are not well sealed, by the rising of heated indoor air, and accumulate due to limited circulation.
The unit of measurement for natural radioactivity is usually the picocurie (pCi), which is roughly equivalent to the decay of two atoms per minute. Radon levels in air are usually expressed as picocuries per liter of air (pCi/L). A typical value for outdoor air is about 0.4 pCi/L, but indoor levels tend to be higher, with 1.3 pCi/L considered average. A level of 4 pCi/L or higher is considered a health hazard.